The present invention relates generally to digital cameras and more specifically to a digital camera that has a photo sensor that has an array of photo-cells that detect non-visible light, embedded in the array of photo-cells that detect only visible light. Using the visible light photocells in conjunction with the non-visible photocells, the digital camera can determine the type of illuminant for the scene.
When capturing an image with a digital camera, the source of the illumination for the scene affects the colors captured with the camera. For indoor scenes the illumination source can vary widely and can include a tungsten bulb, halogen lamps, fluorescent lamps, sunlight coming in through a window, or even a xenon light. Each of these types of light sources has a different spectral energy distribution. The types of light sources that create light using a filament glowing at a high temperature (for example tungsten bulbs) are typically characterized by a color temperature defined as a Planckian radiator with a temperature of 50 degrees higher than the filament of the light (see FIG. 1). The sun can also be characterized as a Planckian radiator but the loss of some wavelengths through scattering and absorption in the atmosphere causes significant differences from the Plankian radiator at those wavelengths. Because of the variation in the spectral power distribution of the sun, standard spectral power distribution curves have been developed. One of the standard curves is called D65 corresponding to a color temperature of 6500K (see FIG. 2). Clouds in the sky can also affect the spectral distribution of energy reaching the scene from the sun. The time of day also affects the color temperature of the sun (noon vs. sunrise). The color temperature can be affected by whether the object is in direct sun light or in shadows.
The types of light sources that excite a phosphor layer that then fluoresces (for example fluorescent lamps and xenon lamps) tend to have spectral distributions that are unique to the phosphors in the lamp (see FIG. 3) in combination with the mercury vapor spectrum.
Each of these light sources has a different spectral power distribution that affects the colors captured in a scene by a camera. For example when you have a white object illuminated by a tungsten bulb the white object will appear yellow in the scene captured by the camera. This is because the tungsten bulb does not produce much blue light. A white object is an object that reflects a similar amount of the red, green and blue light that hits the object. When a white object is illuminated by a tungsten bulb more red light is hitting the object than blue light and therefore more red light is reflected, causing the object to look yellow to the camera. The human eye adjusts to different illuminates and compensates for the color shift but a camera records the actual light in the scene.
Fortunately these color shifts caused by the illumination source can be corrected. This correction is typically called white balancing. For proper white balancing the illuminant of the scene must be known. There are a number of methods currently used to try to determine the scene illuminant to be used in white balancing.
One method looks for the brightest point in a scene and assumes that it should be white. The brightest point is then adjusted until it is white and then this adjustment is used to balance the rest of the scene. This method operates on the assumption that the brightest point in a scene is from a white object or from a specular reflection. For example the specular reflection coming from a car windshield. Obviously not all scenes have the brightest point as a specular reflection or a white object. When this method is used on a scene with a non-white object that is the brightest point in the scene it can result in significant color miss-match. Another method of white balancing adjusts the image until the sum of all the areas in the image adds up to a neutral gray. Both of these methods operate on assumptions about the content of the scene.
Another method uses a correlation matrix memory to map the image data onto color image data under a number of different illuminants. This method is described in U.S. Pat. No. 6,038,399 that is hereby incorporated by reference. When using this method the image data needs to be mapped onto the color data for all potential illuminants. Mapping the image data onto each of the potential illuminants is a computational process. If the set of potential illuminants could be narrowed to the type of illuminant (for example daylight) the amount of computation, and therefore the speed could be reduced. Therefore there is a need for a system that can determine the illumination type for an image in a scene.
A method of determining the illuminant type in an image. Using visible light photocells in conjunction with non-visible or Infered photocells, a digital camera can determine the type of illuminant for the scene.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.